The fabrication of optical fibers includes heating at least an end portion of an optical fiber glass preform within a fiber drawing furnace and drawing a thin glass fiber from an end portion of the preform. The drawn fiber then may be subjected to at least one coating, such as at least one polymer coating. For example, the procedure described by F. V. DiMarcello, C. R. Kurkjian and J. C. Williams, "Fiber Drawings and Strength Properties" in Optical Fiber Communications, Vol. 1, pp. 179-248, T. Li, ed., Academic Press, Inc., 1985, may be utilized to draw the fiber from a preform.
A critical parameter in the production of optical waveguide fibers is the tension within the fiber during the drawing process, and, in particular, the tension in the region between the hot zone inside the fiber drawing furnace and the first fiber coating. The magnitude of this tension affects the final properties of the fiber including the fiber's diameter, ultimate strength and its optical properties.
The temperature of the fiber preform is typically not monitored while inside the fiber drawing furnace. In lieu of this measurement, the temperature of the furnace wall is monitored, and is typically held constant by feedback loops. The temperature of the fiber as it is drawn from the preform is determined by the furnace temperature and also by the gas flow through the furnace. This gas flow is due to injected gases in the case of a graphite furnace or upward convection as the heated gases rise in a zirconia furnace. Typically, this flow is controlled by restrictions which can be placed at either end of the furnace, which limit the gas flow through the furnace. The difficulty associated with accurately duplicating the restriction from preform to preform, along with the errors associated with the siting of the pyrometer of the furnace tube, forces an additional measurement of fiber temperature. This is typically accomplished indirectly by measuring the tension required to draw the fiber. In fact, the tension, not fiber temperature, is usually specified as the parameter to be controlled in fiber drawing. Typically, this tension is measured using a 3-wheel strain gauge which contacts the fiber during the start-up of the fiber drawing procedure. Here, two wheels are applied to one side of the fiber and a third wheel is applied to the other side of the fiber. Unfortunately, the tension is very difficult to measure at high line speeds, e.g., 5 m/s and above, since damage imparted to the fiber by the strain gauge wheels causes the fiber to break. At lower line speeds, such as below 3 m/s, the tension of the fiber is lower and allows the fiber to survive the measurement. While fiber tension varies linearly with draw speed and should allow an accurate determination of fiber tension on basis of the low speed readings, any errors in setting the tension at low speeds are magnified by the ratio of the final line speed compared to the measured speed. This problem is addressed by predicting draw tension from a measurement at a lower speed. To confirm predictions and to detect variability during drawing, the fiber tension at the line speed of interest should also be measured. Therefore, a non-contacting measurement is required if the tension is to be measured at high line speeds while avoiding the severe damage that the 3-wheel strain gauge imparts to the fiber at higher line speeds.
One example of a prior art contactless measurement of the draw tension is disclosed in U.S. Pat. No. 4,692,615 issued to Thomas O. Mensah et al. on Sep. 8, 1987. The tension in a moving fiber is monitored by sensing a vibrational motion of the fiber in a direction transverse to the direction in which the fiber is moving, analyzing the vibrational motion by Fourier transform analysis to determine at least one frequency component thereof, and monitoring the determined frequency component so as to monitor the tension in the fiber. However, measuring the frequency response of the fiber to either vibrations within the fiber due to the drawing process or to an intentional perturbation of the fiber position by puffs of air has certain disadvantages. Vibrations in the fiber can be caused by building and apparatus vibrations, preform feed motor instabilities, fiber drawing motor instabilities, polymer coating application instabilities, to mention a few. While some of these vibrations, such as building vibrations, would remain constant in frequency and, therefore, are relatively easy to identify, sources such as motor noises would increase in frequency with draw speed and would be much more difficult to isolate from the fundamental fiber vibration. Also, quick puffs of air which are used to cause additional vibrations of the fiber can cause fiber diameter feedback loops to become unstable and result in fiber diameter excursions.
Another example of prior art contactless measurement is disclosed by C. G. Askins et al. in "Noncontact Measurement of Optical Fiber Draw Tension", Journal of Lightwave Technology, Vol. 9, No. 8, August 1991, pages 945-947. This technique utilizes analysis of fundamental resonant frequency of the length of fiber between the neckdown region in the furnace and the polymer coating die (station) to determine the tension in the fiber. This technique is subject to disadvantages similar to those in the Thomas Mensah patent, except for the recognition of the existence of frequency components attributed to other sources, and cognizant increase in the fundamental frequency of oscillation of the fiber and reduction of the effects of the other frequency components by means of puffs of nitrogen gas. The process could be useful for known and stable other vibrations but could become difficult if these vibrations are variable or a new set of vibrations is present.
Therefore, a much simpler contactless tension monitoring is needed.